In the art, various earphones are known, which employ passive noise reduction (PNR) to reduce the amount of acoustic noise reaching the wearer's ears. PNR is typically achieved by acoustic dampening in structural components, such as earphone shells and ear cushions. It is further known to combine PNR with active noise cancelling (ANC) that actively counteracts acoustic noise approaching the wearer's ears, thereby attempting to cancel out and thus remove the noise from the sound reaching the ears. ANC is typically achieved by controlling the output of a driver in the earphone such that it counteracts the residual noise that escapes the PNR.
PNR is generally effective at frequencies above about 1 kHz, while the effect decreases towards lower frequencies and is practically non-existing at frequencies below about 100 Hz. Conversely, ANC is generally effective in the frequency range below about 1 kHz, while it is difficult to achieve good results for higher frequencies. Noise reduction using a combination of PNR and ANC can thus in principle be made effective within the entire audio frequency range. At the same time, however, many earphones are intended to reproduce audio in the entire audible frequency range, which includes reproduction of low-frequency sounds. This presents a challenge to the earphone designer, because acoustic output at low frequencies generally requires an acoustically open earphone, while effective PNR requires an acoustically closed earphone.
U.S. Pat. No. 6,831,984 B2 discloses a solution to this problem in a headset. The headset includes an earcup enclosing a front cavity and a back cavity separated by a divider. A driver with a diaphragm is mounted in the divider between the front and back cavity. The headset further includes a circumaural sealing pad constructed and arranged to effectively seal the front cavity to the head of a person. A port and a resistive opening in parallel intercouple the interior and exterior of the enclosure through a wall of the back cavity. The acoustic mass of the port and the compliance of the back cavity are tuned to a resonance frequency of about 300 Hz. This causes the back cavity to behave closed above 300 Hz and open below this frequency. The resistive opening dampens the port resonance, which would otherwise cause a narrow dip at 300 Hz in the sound output to the ear. A disadvantage of the disclosed solution is that sound waves with a frequency above the resonance frequency can nevertheless enter the back cavity through the port and through the resistive opening, partly due to natural resonances in the port, which decreases the effect of the PNR provided by the earcup. The result is a reduction in the total noise reduction, primarily in a broad frequency region around and above 1 kHz where the transition from PNR to ANC takes place.
U.S. Pat. No. 5,497,427 discloses an alternative solution in a similar headphone, which allows the user to manually switch between an open and a closed configuration. Instead of a port and a resistive opening, a diaphragm without a driver is arranged to cover a window hole in the wall of the back cavity. The diaphragm is tuned to resonate at around 1300 Hz. A lid member can be manually attached to shut the window hole or be removed to open the window hole. With the lid member removed, sound is not attenuated in the low-pitched sound range, and external sounds can also be heard. Therefore, the user can hear reproduced sound of music and the like while playing sports outdoors or taking a walk. The headphone can also be used as a general closed type headphone by using it with the window hole closed by shutting it with the lid member. In this case, some attenuation of the sound in the low-pitched sound range is present, while the external sound is scarcely heard. Obviously, the disclosed alternative solution does not allow simultaneously achieving high-level low-frequency sound and effective PNR.
Japanese Patent 4826399 discloses a variant of the alternative solution, which allows the user to manually move a braking member between a “closed” position wherein the braking member contacts the diaphragm and thus prevents the diaphragm from vibrating and an “open” position wherein the braking member does not contact the diaphragm and thus allows the diaphragm to vibrate. The diaphragm is tuned to resonate at around 1-2 kHz.